Re: [virtio-comment] Re: [PATCH 0/2] Selective queue enabling

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

On 6/14/2023 5:54 AM, Parav Pandit wrote:
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Tuesday, June 13, 2023 5:48 PM
> > > Why queue_reset is needed here?
> > > I don't see spec saying that queue_enable must be done only once for a given
> > Q before DRIVER_OK.
> >
> > I see you found it.
> >
> > > > - program another queue address, size
> > > >
> > > > Finally we are decided, let's start operating:
> > > >
> > > > - DRIVER_OK
> > > >
> > > >
> > > > I don't see how it's possible with just queue_enable.
> > > >
> > > >
> > > > > It is not explicitly mentioned in the spec that one can setup the
> > > > > queue using
> > > > queue reset instead of queue enable.
> > > > > As Jason mentioned it is implementation specific, one device
> > > > > supports it and
> > > > one doesn't.
> > > > > Hence, it will break on those devices which doesnt support it.
> > > > Setup? No, and spec explicitly says to setup one has to use
> > > > queue_enable. But this is not what we discussed with Stefano here.
> > > The above sequence you described is not well documented.
> > It's the standard sequence for queue reset, is it not?
> >
> > For example:
> >
> > 	Virtqueue reset is divided into two parts. The driver first resets a queue
> > and
> > 	can afterwards optionally re-enable it.
> >
> >
> > it seems clear that queue has to be enabled before being reset.
> Yes, it is clear.
>   
> > I'm not sure what's broken.
> If a device has assumed that queue_reset must be done after DRIVER_OK stage because until that point the device is not "live",
> Such device needs the fix.
I think queue_reset just resets the queue and its fields including the 
queue_enable. DRIVER_OK is not a must for performing a queue_reset,
My understanding is:
- before DRIVER_OK, the driver can re-config the queue anytime, doesn't 
need a reset_queue
- after DRIVER_OK, the driver has to reset the queue to re-config the 
queue. Because a queue is considered alive when both DRIVER_OK && 
queue_enable,
and resetting a queue also set queue_enable to zero.

At the end of the migration, hypervisor should reset the queues and 
config the queues with the guest configurations and enable the queues, 
right?

Thanks